Because of the use of traditional lead (Pb) shot has been outlawed for waterfowl hunting in the U.S., Canada, U.K. and other countries, much effort has been devoted to identifying a suitable substitute. To be fully satisfactory, alternative shot must possess the following attributes:                a) The material should have density similar to that of lead (Pb) shot, typically 11.0 g/cm3.        b) The material must not cause physiological problems in wildlife which may ingest spent shot from the ground or water.        c) The material must not cause significant damage to shotgun barrels.        d) Shot must possess sufficient strength, rigidity and toughness to adequately withstand “set-back” forces associated with firing and to penetrate the target effectively without shattering or excessively deforming.        e) For purposes of game law enforcement, shot material should preferably be magnetic to easily differentiate it from illegal lead shot.        f) Material used for shot must be economical to obtain and fabricate into spherical product.        
None of the alternative shot types currently available conforms to all of the above criteria. Current products in the USA include shot made of steel, bismuth alloy, iron-tungsten alloy and tungsten-polymer composite. Each of these will be reviewed and critiqued in the following discussion, followed by a review of other prior art which has not yet become commercialized.
Steel Shot
The most widely used alternative shot is carbon steel, in spite of the fact that its density is quite low (about 7.9 g/cm3) in comparison with that of lead shot (about 11.0 g/cm3). Inarguable principles of physics and engineering establish that an object of lower density, when moving through a fluid (such as air), will carry less energy at any given velocity, and experience more rapid loss of velocity (due to drag forces) than an object of higher density of the same size and shape. Shotshell manufacturers have employed special powders to increase steel shot velocity, in an attempt to ameliorate its inferior ballistic properties. The “hotter” powders unfortunately create higher pressures within the gun barrel. Safety considerations have therefore prompted shotshell manufacturers to recommend that steel shells only be fired in certain types of modern, high-strength shotguns.
There is also a significant negative impact of steel shot on the very same wildlife which the outlawing of lead is intended to preserve. The inferior ballistics of steel shot, in the hands of the general public, has resulted in higher rates of “crippling” shots. The January, 1997 issue of American Hunter refers to “Goose hunters accustomed to shooting traditional lead shot tend to attempt to shoot waterfowl at the same distances as they have always considered to be “in range.” Another approach taken by steel shotshell manufacturers has been to simply substitute larger steel shot for traditional lead shot sizes, in order to provide equivalent mass.
This practice has the obvious disadvantage that there are fewer shots in any given shell. The “pattern density” of the cloud of shot is lower at any given distance from the point of firing. This sparse pattern again increases the probability that birds will be crippled, rather than harvested for consumption. In summary, a statement by the Shooting Editor of Outdoor Life Magazine, Jim Carmichel, is quoted: “. . . steel shot has generally been considered only a quick fix in the search for the ultimate shot pellet.” (April, 1997 issue, page 73).
Bismuth Shot (U.S. Pat. No. 4,949,644 to Brown)
Bismuth alloy shotshells are currently marketed in the USA at approximately three times the cost of steel shells, an indication of how desperate consumers are to obtain improved performance. Unfortunately, bismuth alloys are not equivalent to lead in density (about 9.4 g/cm3 vs. 11.0 g/cm3), although somewhat more dense than steel (7.9 g/cm3). In addition to this shortcoming, bismuth alloys are inherently brittle and therefore tend to fracture and disintegrate upon impact (January, 1998 issue of Gun Tests). As fracture surfaces form in the shot, energy is lost which would otherwise be available to enhance penetration of the target. In this instance, it is even likely that all the increased energy gained by having higher density than steel is lost as fracture occurs. Finally, it should be noted that bismuth is non-magnetic and cannot be readily distinguished from illegal lead shot by game officers in the field.
Iron-Tungsten Shot (U.S. Pat. Nos. 5,264,022, 5,527,376 and 5,713,981 Assigned to Teledyne Industries, Inc.)
A more recent product which began to be marketed in the USA in 1997 is a shotshell containing binary iron-tungsten alloy shot (60% Fe—40% W, by weight). Because the Fe—W is very hard (about Rockwell C50), and therefore must be ground with ceramic abrasives (alumina, silicon-carbide, diamond, etc.), particles of which become imbedded in the shot surface, this type of shot will result in severe damage in all gun barrels unless the shot is encapsulated in a special “overlapping double-wall” plastic shot-cup of heavy construction. Even with this precautionary design, the manufacturer prints a clear message on each box of product disclaiming any responsibility for gun barrel damage or personal injury. Although controversial, one current theory is that it is possible for a few shot to rebound forward out of the plastic cylinder upon firing and to thereby contact the unprotected steel barrel. The consequences of forming longitudinal scratches on the barrel are that stresses produced by the expanding explosive gases will be concentrated in the regions around the scratches. A primary concern is that these stresses may be sufficiently high to cause catastrophic bursting of the barrel.
Whether adequately protective or not, the special plastic shot-cup (or “wad”) creates another significant problem. The wad must be made of plastic tubing so thick as to make it impossible to load quantities of shot equivalent to those of traditional lead shells. For example, Fe—W shells of 2¾-inch length for 12-gauge guns contain only 1.0 ounce of shot versus 1⅛ to 1¼ ounces in corresponding lead or steel shells. The deficient pellet numbers result in correspondingly sparse pattern densities, the same problem encountered in substituting larger steel shot for traditional lead sizes, as mentioned previously.
Although more dense than bismuth shot, Fe—W shot currently marketed is still considerably less dense than lead shot (about 10.2-10.5 g/cm3 vs. 11.0 g/cm3). When this fact is combined with the lower pattern densities, the purported advantages of Fe—W shot over steel shot become questionable.
Finally, problems associated with manufacturability, and their adverse effects on product cost, are relatively severe. The constituent phases in Fe—W alloys cause the shot to be so hard and brittle as to be impossible to forge or swage these alloys into rods, or even to shape them compressively into spheres. Although the referenced patents claim Fe—W shot can be made by casting, the inherent brittleness and high melting temperatures of these alloys caused cracking to occur during rapid cooling. Cracking also plagued the process of compressive grinding, which was tried as a means of rounding the generally asymmetrical shot. Consequently, the shot actually being produced and marketed must be made by an expensive powder metallurgical method. Even with this approach, only larger shot sizes (“BB” 0.180-inch-diameter, and “#2”0.150-inch-diameter) are being produced at present. This is due to the fact that powder processing costs increase exponentially as shot sizes decrease. Furthermore, the fragility of compaction tooling becomes a limiting factor as shot size decreases. Shot sizes #4 (0.130-inch), #5 (0.120-inch), #6 (0.110-inch) and #7½ (0.095-inch), traditionally preferred for hunting all but the very largest game birds (such as geese), are unavailable for these reasons.
Attempts to increase Fe—W shot densities to be equivalent to lead shot are frustrated by the fact that elevating tungsten content not only raises material costs but further exacerbates fabricability problems. As in the case of bismuth shot, Fe—W shells are about three times as expensive as steel shells, thereby rendering them unaffordable by the average sportsman. Unlike steel shot, which can be obtained by the average citizen to reload his own sporting ammunition, Fe—W shot and the special plastic wads which make it allegedly safe to use have not been made available to the public for reloading (April/May, 1995 issue of Wildfowl Magazine).
Tungsten-Polymer Shot
A new version of an older idea (U.S. Pat. No. 4,949,645 to Hayward et al.) is currently proposed for the U.S. market in 1998-1999 (January/February, 1995 issue of Ducks Unlimited Magazine and March, 1998 issue of Petersen's Shotguns). This shot material is a composite of tungsten powder and a powdered polymer (e.g., nylon, polyethylene, et al.). Mixtures of these two constituents are formed into spheres of cured composite, the polymer “glue” being the continuous phase and the tungsten powder particles the discontinuous phase. By virtue of its weak polymer-to-metal bonds, the material will reportedly not damage gun barrels. It is this very “weakness,” however, which is one of the undesirable features of tungsten-polymer shot. Rigidity and strength are important material properties which affect the ability of shot to (1) penetrate the target effectively, and (2) remain spherical during launching and flight.
The penetrability factor can be easily understood by considering the behavior of a rubber bullet (used, for example, by police). The projectile does not penetrate well because its kinetic energy is absorbed and dissipated by its own deformation. Rigidity, as used here, is measured by a material property value known as elastic modulus. Because the elastic moduli of all organic polymers are far lower than those of metals, the subject composite materials are, as expected, less rigid than steel, Fe—W, et al. The second factor is important when a different type of shot distortion/deformation occurs which causes loss of sphericity, thereby degrading shot pattern density and uniformity. During firing, the shot experiences high compressive “set-back” forces. Materials which are relatively weak (i.e., low in yield strength), undergo various degrees of permanent distortion, referred to as “plastic deformation.” Any loss of sphericity will result in erratic flight paths of shot and will therefore produce undesirable pattern uniformity.
Another disadvantage of tungsten-polymer shot is one of economics. Because polymers are much lower in density than common metals such as iron, a composite density equivalent to that of lead shot (11.0 g/cm3) can only be attained by using high concentrations (e.g., 95%) of costly tungsten powder.
As in the case of bismuth, tungsten-polymer shot is non-magnetic, making it difficult for law enforcement to distinguish it from illegal lead shot.
Other Prior Art
A number of proposed alternative shot materials demand the use of expensive powders as input to processes which include mixing, pressing, sintering and sizing. These processes are expensive and difficult to control, beginning with the challenge of characterizing the input powder particle sizes, distributions and shapes. Many of these processes require the use of special atmospheres such as hydrogen or vacuum to protect constituents such as tungsten powder against oxidation during high-temperature processing. Alternative shot materials in this category include U.S. Pat. No. 4,784,690 to Mullendore et al. As in the case of Fe—W shot, such processes can, at the most, only be expected to be economically feasible for the larger shot sizes, which have limited usefulness.
Other proposed shot materials include significant concentrations of lead as a specified ingredient. Recent rulings by the U.S. Fish and Wildlife Service have outlawed the use of any shot material containing more than 1.0% lead. This action has eliminated consideration of proposed materials described in a variety of U.S. Patents: U.S. Pat. No. 2,995,090 to Daubenspeck; U.S. Pat. No. 3,123,003 to Lange, Jr. et al.; U.S. Pat. No. 4,027,594 to Olin; U.S. Pat. No. 4,428,295 to Urs; U.S. Pat. No. 4,881,465 to Hooper; and U.S. Pat. No. 5,088,415 to Huffman et al. are examples.
Even materials which are lower in density than steel have been proposed for alternative shot. Examples are zinc (7.14 g/cm3) and tin (7.3 g/cm3), the latter being reported in the Sep. 4, 1997 issue of American Metals Market. Such materials certainly offer no improvement in ballistic properties over those of steel shot.
Finally, a general criticism which can be made for all so-called “high-density, non-toxic” shotshells presently available to the public is that they are approximately three times as expensive as even “premium grade” steel shotshells. This fact discourages the average hunter from actually purchasing these products, thereby frustrating agencies and individuals who are attempting to find a suitable substitute for traditional shot. One of several preferred objectives of the present invention is to place emphasis on materials and processes which are more economical than those required by other non-toxic, high-density shot options.
Objects and Advantages
Accordingly, the present invention addresses and solves each of the problems associated with other alternative shot types. Several objectives of the present invention, which may be achieved individually or in groups according to various aspects of the present invention, are:                a) to provide a shot material which, unlike conventional Fe—W alloys, is castable and formable and therefore able to be manufactured by conventional processes;        b) to provide a shot material which, unlike Bi and Fe—W products currently available, is fully as dense as lead alloy (11.0 g/cm3) or higher;        c) to provide a shot material which, unlike Fe—W and high-carbon steel, is much softer than gun barrel steels, thereby reducing or eliminating damage;        d) to provide a shot material which, is non-toxic to wildlife and the environment;        e) to provide a shot material which, if desired, can be made magnetic for game-law purposes, unlike Bi and tungsten-polymer;        f) to provide a tough shot material which will not fracture or disintegrate upon impact;        g) to provide a shot material which, unlike Bi, tungsten-polymer and low-carbon steel, is strong enough to withstand firing without distorting (but soft enough to minimize gun barrel damage);        h) to provide a shot material which, by virtue of its softness, is suitable for use with conventional plastic wads used for low-carbon steel, thereby making it possible for private parties to load and use it; and        i) to provide a shot material which, by virtue of its ferromagnetic properties, may be readily salvaged for reuse, unlike Bi and tungsten-polymer shot; and        j) to provide a castable material having a density in the range of 8-10.5 g/cc; and        k) to provide a castable material having a density in the range of 10.5-15 g/cc.        
A further objective is to provide a shot material which, because it may be salvaged and reused, will enable groups and individuals to offset initial shot costs by recycling. This will allow W-containing shot to be economical for recreational shooting (e.g., trap, skeet, and sporting clays). Devices and methods for performing the actual salvage operations are also suggested in the present invention.
Still further, a shot material ultimately is provided which, in its preferred embodiment of alloy melting, casting, and fabrication, can use virtually any source of tungsten as input material. This includes, but is not limited to, virgin tungsten, scrap tungsten, ferrotungsten, tungsten alloys, tungsten-carbide, et al. It also includes a novel consideration of utilizing a unique, less-expensive type of ferrotungsten directly reduced from forms of the mineral “wolframite,” (FeMn)WO4.
In connection with shellshot formed according to the present invention, an objective is to produce tungsten alloys for shot which, unlike conventional iron-tungsten alloys, are castable and ductile enough to be formable by conventional processes and equipment, and which can utilize less expensive sources and types of W. Toward this end, a scientific approach, using sound principles of metallurgy and physics, has been used to solve a specific set of problems.